Disk drives, particularly but not necessarily magnetic hard disk drives, record blocks of user data in concentric data tracks defined on major surfaces of a rotating rigid magnetic disk. A head structure supports and positions a data transducer head at each selected data track of an associated data storage surface in order to carry out reading or writing operations. In order to seek to, and then follow, each selected data track, the head positioning structure must perform head tracking operations, typically by using a form of servo control loop. Most prevalently, servo loops derive head position information from servo sectors which are interspersed or "embedded" within the data tracks. As each servo sector is encountered, user data transfer operations are suspended, and the head then senses prerecorded patterns of magnetic flux transitions which may include a unique servo address mark pattern to confirm that the head is reading from the servo sector, a coarse track number which is typically Gray coded, and fine position information which serves as a head position vernier within each track.
Within disk drives having a multiplicity of concentric data tracks defined on a rotating magnetic storage surface, head tracking has conventionally been carried out by asynchronously detecting Gray code track number information recorded within embedded servo sectors in order to provide coarse head position information to a head position servo loop. The servo control loop employed the coarse head position information, particularly during track seeking operations to control head trajectory from a departure track to a destination track. Once the destination track was reached, further information, known as "fine information" was needed to provide fine positional adjustments to the head structure, to be sure that the head was precisely following a centerline of the destination track.
In prior peak detection recording channels, the fine head position information has been embedded within the servo sectors in generally two different ways. One method employs a dibit, tribit or quadrabit pattern in which aligned flux transitions recorded in half-track-width patterns are summed and differenced within the magnetic data head structure. A discussion of the synchronous bit fine servo approach is provided in U.S. Pat. No. 4,101,942 to Jacques entitled: "Track Following Servo System and Track Following Code". Another approach in which between-track positions are capable of being resolved is given in U.S. Pat. No. 4,032,984 to Kaser et al., entitled: "Transducer Positioning System for Providing Both Coarse and Fine Positioning". One drawback of these prior approaches was that they were particularly susceptible to noise interfering with the single or few flux transitions providing the fine position information. Another drawback was that they employed asynchronous peak detection techniques which required that the pulses carrying the head position information be spaced sufficiently far apart so as to eliminate any possibility of intersymbol interference from adjacent pulses, and the resultant servo patterns required a considerable space within the servo sector format. Finally, even though some off-track position resolving capability existed, as described in the referenced Kaser et al. patent, the off-track signal was highly transitional and marginal, and was not capable of providing a robust, reliable position value.
Another prior peak detection servo pattern recorded in embedded servo sectors has been the plural burst pattern. In this approach, at least two (A and B) servo bursts are recorded in a radially offset, circumferentially staggered arrangement within each servo sector. These patterns were sequentially read by the data transducer head, and resultant relative amplitudes were sampled, peak detected, integrated and then passed into a servo data channel which asynchronously filtered the e.g. two, three or four separate and sequentially read sine wave burst fields (herein the "A", "B" and "C" bursts) using analog techniques. A position error signal (PES) was derived by calculating the difference in playback amplitudes of a selected pair of the available bursts. Since each of the selected burst pairs is offset in radial position from the other burst pair, when the head is on-track, equal relative burst amplitudes are read from each of two bursts, e.g. the A and C bursts. For thin film read heads, equal amplitudes are read when approximately one half of the head is over each burst.
In one prior example, when the head is not close to the track centerline, as during track settling following a seeking operation, the head makes use of a third or B burst to obtain an accurate measurement of the off-track distance. For example, in one preferred pattern, depicted and described in commonly assigned U.S. Pat. No. 5,170,299 to Moon, entitled: "Edge Servo for Disk Drive Head Positioner", burst amplitudes from bursts B and C will be equal when the head is reading exactly 1/4 track width off track centerline. The disclosure of U.S. Pat. No. 5,170,299 is hereby incorporated by reference.
There are several drawbacks stemming from the prior approach. One drawback is that the three servo bursts, A, B and C, and the gaps separating them require considerable area within each servo sector, thereby increasing the servo overhead and reducing the amount of user data that can otherwise be recorded on the storage surface.
Another drawback is the requirement for separate analog circuitry for servo as in the case of a partial response, maximum likelihood ("PRML") read channel. While the digital peak detection process set forth in commonly assigned U.S. Pat. No. 5,321,559 to Nguyen et al., entitled: "Detection of Embedded Servo Sector Data with PRML Channel in Disk Drive", improved somewhat upon prior approaches by using the channel A/D for conversion of peak values, a need remained for a separate analog peak detection path, a path control and three burst sample and hold circuits. These added circuits and complexities have added cost within the disk drive system. Also, the prior approach used the conventional A, B, and C burst arrangement described above which is not particularly efficient or compact in terms of disk storage space. The disclosure of U.S. Pat. No. 5,321,559 is hereby incorporated by reference. A different asynchronous servo address mark detection method was described in U.S. Pat. No. 5,255,131 to Coker et al., entitled: "Asynchronous Servo Identification/Address Mark Detection for PRML Disk Drive System".
Synchronous servo detection methods within synchronous user data detection channels such as PRML hold the promise of increased burst accuracy as well as more efficient embedded servo sector patterns. In addition, by eliminating separate analog burst sampling and detection circuitry including in some cases a separate A/D converter for quantizing servo bursts, added cost can be reduced. Also, by using synchronous servo detection, the burst fields can be made more compact, thereby increasing the storage area available for user data.
One known method for performing synchronous servo detection is by employing the PRML read channel. A slightly modified version of data read mode can be used to detect servo Gray code and obtain the A, B, and/or C burst information, as explained in the related, commonly assigned, copending Fisher U.S. patent application Ser. No. 08/174,895 filed on Dec. 23, 1993, entitled: "PRML Sampled Data Channel Synchronous Servo Detector", now U.S. Pat. No. 5,384,671, the disclosure thereof being incorporated herein by reference. In this prior approach, the three burst servo scheme (or a four burst scheme for MR read heads) was retained. The servo burst sinewave values read by the head were low pass filtered, then digitized by the read channel A/D converter and finally accumulated. Due to the unpredictability of the burst amplitude at any given time (burst values are acquired to ascertain track position), PRML timing and gain loops were not adapted during measurement of the A, B and C burst relative amplitudes. It was thus assumed in this prior approach that the timing and gain loops would "coast" over the servo burst regions, or that additional synchronization fields would be inserted between the bursts to enable resynchronization of the timing and gain loops thereby adding to the servo overhead on the disk storage surface. Thus, this earlier solution was less than optimal in performance and continued to be wasteful in disk data storage area. Even though three distinct bursts were recorded in each servo sector, only two different values were of interest to the servo head position circuitry at any given time: the on-track position error signal A-C, and the off-track position error signal B-C.
Thus, a hitherto unsolved need has remained for a more compact and efficient servo burst pattern from which position error signals may be detected with synchronous sampling techniques in a more efficient manner than before.